1. Field of the Invention
The present invention relates generally to coaxial cables. More particularly, the present invention relates to coaxial cables which permit the signal carried by the cable to radiate through apertures in the outer conductor.
2. Discussion of Related Art
In the prior art, coaxial cables are formed by disposing an inner-conductor within a dielectric material. The dielectric material and inner-conductor are disposed within an outer-conductor. The dielectric material is usually a foam material, but may be air. The outer-conductor is typically covered by an outer jacket.
Radiating coaxial cables are designed so as to permit the signal carried by the radiating coaxial cable to radiate through one or more apertures formed in the outer-conductor. For example in a patent issued to Allebone (U.S. Pat. No. 4,325,039), circular apertures are formed in the outer conductor. Another example of a radiating coaxial cable is provided by U.S. Pat. No. 5,339,058 issued to Lique which discloses a coaxial cable with a slot in the outer-conductor for radiating the signal carried by the coaxial cable.
A common requirement of radiating coaxial cable is resistance to flame propagation. In radiating coaxial cable having a foam dielectric, flame propagation may be encouraged if the dielectric melts and escapes through the radiating apertures. In order to provide the necessary fire protection, radiating coaxial cables employ a fire retardant material in the outer jacket. Other prior art designs employ a barrier tape, installed between the outer-conductor and the outer jacket or between the dielectric material and the outer-conductor, which contains the foam dielectric at melt temperatures. Examples of coaxial cables using barrier tape are found at U.S. Pat. Nos. 4,800,351 and 5,422,614 issued to Rampalli et al.
In the case where the outer-conductor of a radiating coaxial cable is corrugated, it is common to form the radiating apertures in the outer-conductor by passing the outer-conductor through a milling machine which removes part of the outer-conductor in order to form a radiating aperture. In a continuous milling process, the milling tool is positioned at a fixed distance from the center-line of the outer-conductor, and the outer-conductor is fed axially into the milling machine so that as the crests of the corrugations pass by the milling tool, a series of oval apertures are produced in the crest of each corrugation of the outer-conductor. U.S. Pat. No. 5,422,614 discloses a corrugated outer-conductor having oval apertures in the corrugation crests which may have been produced via the continuous milling process.
In the continuously milled, corrugated outer conductor, the apertures are spaced apart according to the spacing of the corrugations. However, in some applications, this aperture spacing is not desirable. As compared to an intermittently milled cable that does not have apertures at every corrugation, the continuously milled cable has a larger attenuation of the signal for a desired coupling loss. Also, continuously milled cables have a larger direct current resistance for a desired coupling loss than intermittently milled cables.
On the other hand, coaxial cable formed by intermittently milling the outer-conductor tends to have voltage standing-wave ratio ("VSWR") spikes. Intermittently milled coaxial cable is also more expensive to manufacture than continuously milled radiating coaxial cable, especially when the spacing between apertures is not constant, which is sometimes required in order to control VSWR spikes.